A carelessly discarded cigarette, a downed power line, a car's backfire or a chainsaw's pull. Just about anything could have started any one of the wildfires now tearing through Northern California, authorities said.

"Every spark is going to ignite a fire," said Ken Pimlott, the state's top firefighter. He said the risk remains "extreme for new starts."

Pimlott said Tuesday that investigators are looking into the causes, but no determination has been made at any of the sites of major wildfires blazing in Northern California. Authorities said 22 were burning Wednesday.

Pimlott, the California Department of Forestry and Fire Protection director, said "98 percent" of all wildfires are started by people, and it's unlikely lightning is to blame for any of the fires that exploded overnight Sunday, killing at least 21 people so far.

California's most dangerous wildfire season comes in autumn, when summer heat and insects have left brush dead and dried out, and winds are especially hot, dry and strong.

"This is traditionally California's worst time for fires," Pimlott said.

Pimlott said firefighters typically respond to 300 blazes a week during this season, but nearly all are extinguished quickly and with minimal damage. It's unusual to have many major fires burning at once, he said.

However, conditions were ripe for wildfires in California wine country after record rains last winter created an abundance of dry vegetation, which combined with low humidity and unusually high winds gusting to 79 mph to create fast-moving infernos.

Pimlott told reporters Wednesday that the state was still feeling the effects of five years of drought and that the winter rain was gone, leaving behind critically dry vegetation to fuel fires.

None of the major fires has been contained. They are spread over a 200-mile region north of San Francisco from Napa in the south to Redding in the north, taxing firefighting resources.

Napa County Fire Chief Barry Biermann said fires had been moving too fast and unpredictably for firefighters to attack directly.

"The winds were extremely erratic during those conditions of high winds and a lot of things happened," Biermann said.

He and others said resources are stretched thin as firefighters battle so many major blazes simultaneously.

California Office of Emergency Services Director Mark Ghilarducci said thousands of firefighters, law enforcement officials and others are responding. Airplanes are helping and fresh firefighters from Southern California and Nevada are streaming in to help.

The U.S. Department of Defense is sending a large drone to help map the fires and assess damage. The California National Guard is also providing gasoline to firefighters and other first responders because many service stations in the area are without power and unable to pump fuel.

The biggest and most devastating fire is burning in Santa Rosa, a city of 175,000 people 45 miles north of San Francisco. A fire there swept through several neighborhoods and business districts. Many residents had only minutes to flee. Many of the fatalities so far have occurred in and near Santa Rosa.